(Adapted from the Applicant's Abstract) The ultimate aim of this research is to determine the potential role of dystroglycan dysfunction in congenital heart disease. The overall objective of this project is to investigate the function of dystroglycan in cardiac embryogenesis. Extracellular matrix components and receptors are critical for the morphogenesis of the heart and are excellent candidates for involvement in congenital heart disease including atrioventricular canal defects and perimembranous interventricular septal defects. Dystroglycan is a broadly expressed cell surface extracellular matrix receptor that is linked to the cytoskeleton. Recent studies have indicated that dystroglycan plays a critical role in myogenesis and organogenesis. To investigate the role of dystroglycan in developmental processes, the investigators disrupted the dystroglycan gene in the mouse. The null mutation results in early embryonic lethality, prior to the onset of gastrulation. This phenotype stems from the failed development of Reichert's membrane, an extraembryonic basement membrane structure in which dystroglycan is also expressed. From these studies, dystroglycan seems to be required for either the anchorage of cells to the extracellular matrix or the assembly of networks of extracellular matrix proteins. Dystroglycan's involvement with extracellular matrix and its expression in various cell types suggest that it could be involved in many aspects of heart morphogenesis. To begin to address the role of dystroglycan in cardiac embryogenesis, the investigators will first define the developmental expression of dystroglycan in the heart (Specific Aim 1). To directly examine dystroglycan's function in heart embryogenesis, the investigators have proposed experiments to circumvent the early lethality of the dystroglycan null mutation so that the investigators may analyze dystroglycan's role later during heart development (Specific Aims 2 to 4). The first specific aim will utilize normal mouse embryos to establish a map of the spatial and temporal pattern of expression of dystroglycan during heart development. The second aim will analyze the cellular role of dystroglycan in the development of cardiomyocytes in embryoid bodies. The third aim will use tetraploid complementation in order to rescue the Reichert's membrane defect and thus allow them to examine dystroglycan-null embryos that develop to later stages after the onset of heart development. The fourth aim will be the myocardium-specific disruption of the dystroglycan gene. The goal of the last two specific aims is to examine heart development in embryos that lack dystroglycan in all cells (Specific Aim 3) or just cardiomyocytes (Specific Aim 4). The complementary approaches outlined in these specific aims will yield a new understanding of the role of dystroglycan in heart development and will constitute a foundation for future investigations directed toward the identification of congenital heart disease patients with abnormalities in dystroglycan function.